Heretofore, starters of this type had planetary reduction gears built therein, as shown in FIG. 1. In this figure, 101 designates an armature, and 102 is an armature rotation shaft with a commutator 103 engagingly set thereon at the rear part thereof. To this commutator 103, armature coils 104 are connected.
Brush and holder 105 are disposed to be in contact with the commutator 103, and are secured to a rear bracket 107. 108 is a bearing, and 109 is a yoke of a dc motor which has a plurality of permanent magnets 102a securely set on an internal circumferential surface thereof.
On an end surface of this yoke 109 is a front bracket 111 wherein an internal gear 110, which comprises a planetary reduction gear, is engagingly fit and mounted as shown in the figure. Spur gear 112 and internal gear 110 engage with a plurality of planetary gears 113. Bearing 114 is supported by a supporting pin 115 and is engagingly set to the internal circumferential surface of the planetary gear 113. Flange 116 fixedly supports supporting pin 115 which acts as an arm supporting a planetary reduction gear, with a rotation output shaft 117 fixedly set thereon at the rear. Sleeve bearings 118 and 119 engagingly fit in the rear inner circumferential cavity of the rotation output shaft 117, which bears the front end part of the aforementioned armature rotation shaft 102.
Steel ball 120 is provided for giving and taking the thrust force, and helical spline 121 is formed on the outer circumference of the intermediate part of the rotation output shaft 117, with an overrunning clutch 122 in front and rear slidably splined thereon. A stopper 123 is provided at the front end part of the output shaft 117 for controlling the axial shift of a pinion 124, and a sleeve bearing 125 is provided for bearing the rotation output shaft 117 at its front end part, which is engagingly set on the front end inside surface of the front bracket 111. Lever 126 has a rotation shaft 126a, each end of which is to be engagingly fit on the outer peripheral parts of a plunger 128 of an electromagnetic switch 127 and the overrunning clutch 122, as shown in FIG. 1.
A movable contact 129 is mounted on a rod 131 through an insulator 130, and the rod 131 is movably inserted in a core 132. Fixed contact 133 is fixed by means of a nut 134 to insulate cap 135. Exciting coil 136 urges the plunger 128, which is wound on a bobbin 137 and is contained in a case 138 in that state. A lead wire 139 connects the fixed contact 133 with a brush of the brush-and-holder 105.
The operation of this prior art starter is described below.
By closing a key switch (not shown), an exciting coil 136 of the electromagnetic switch 127 is electrified, whereby the plunger 128 is urged to move backward to push the rod 131 backward, thereby making the movable contact 129 abut on the fixed contact 133. Power is then supplied to the armature coils 104 from the fixed contacts 133 through lead wires 139 and the brush-and-holder 105, causing the armature 101 to produce a turning force. The rotation of the armature 101 is transmitted from the spur gear 112 to the planetary gear 113 and then to the overrunning clutch 122, with the speed reduced by the planetary reduction gear. At this time, the pinion 124 which is engaged with the overrunning clutch 122 is to rationally driven together with the rotation output shaft 117.
The force of plunger 128, urged as described, rotates the lever 126 anticlockwise with the rotation shaft 126a as the center of rotation, causing overrunning clutch 122 to slide forward along an axial line with the pinion 124. Thereby, the pinion 124 engages with, for example, a ring-gear circumferentially provided on a flywheel mounted on a crank shaft of an engine (not shown), to start the engine.
With the conventional engine starter as hereabove described, the electromagnetic switch and the dc motor are parallel-axially composed. Therefore, when the conventional engine starter is mounted on a vehicle, a space for the electromagnetic switch is needed on the vehicle side, exclusive of the engine or the engine section, thus imposing a restriction on the engine layout in the vehicle and causing other problems.
In order to avert such a problem it was proposed to provide a starter unit in a simple form as a mere oblong cylinder, with the electromagnetic switch arranged on one axial end side of the dc motor. According to this proposal, the basic composition is such that the plunger rod of the electromagnetic switch unit or a similar push rod is extended to the rotation output shaft through the internal passage of the armature rotation shaft. Such a starter unit is called a coaxial type starter unit, because the armature rotation shaft of the dc motor and the rod of the electromagnetic switch unit are arranged on a common axial line.
However, when the coaxial type is adopted as hereabove proposed, its overall shape would become a simple oblong cylinder, but this has involved the problem of its total length becoming too long.
In a coaxial type starter, with the electromagnetic switch disposed at the rear of the motor, it is necessary that the armature rotation shaft should be hollowed and the rod of the magnetic switch inserted into the shaft to push the rotation output shaft. In a case however, there is a fear that dust or oil may enter through the armature rotation shaft or brush powders; and thus there is the problem that complete sealing of the contact chamber of the electromagnetic switch is impractical.
Therefore, the conventional coaxial type starter is formed so as to have its contacts installed in separate places thereby resulting in a lengthened dimension of the starter.
In the above-described coaxial type starter, a coil spring for restoring the aforementioned rotation output shaft to its former position after the engine has started is necessary, and there is the problem that depending on where this coil spring is installed the total length of the starter becomes too long, and the additional problem that the composition of the stopper for restricting the forward movement of the aforementioned rotation output shaft becomes complex.
Further, to avoid interference of the end edge part of the front bracket with, for example, the flywheel inside the engine mission housing, the configuration of the flywheel is restricted.
DC motors employed in vehicle starter units and the like also are structured as shown in FIG. 2. Such a conventioned dc motor is equipped with an armature core 101 mounted on a rotation shaft 102, and a commutator 103 is supported on this rotation shaft on one side of armature core 101. This commutator 103 is a so-called face type, having a contacting surface 103a, with brushes 105a of the brush unit 105 formed as a perpendicular surface normal to the axial line of the rotation shaft 1. The commutator 103 comprises a large number of commutator segments 103b insulated from each other with a synthetic resin. To each of these segments 103b, a terminal part 104a of each armature coil is drawn out of the armature core 101 and connected. Accordingly, these terminal parts 104a are closely arranged around the rotation shaft 102 in its circumferential direction, thereby forming a ringed terminal wire group 104.
On the outer circumference of this ringed terminal wire group 104, a fastening ring 200 which is called a "bid" is provided for preventing outward projection of said terminal wire group 104 due to centrifugal force when it is turned together with the armature core 101 and the commutator 103. This bind 200 is generally formed by winding piano wire 200a on the upper surface of an insulating paper 201. In place of the piano wire 200a, a ringed iron plate or a tape impregnated with a resin, or the like, may be used.
In FIG. 2, 105b designates a holder for the brush unit 105, and 202 is a mold part for supporting the commutator 103 on the rotation shaft 102 through an insert 203.
Generally, when a dc motor is utilized as a vehicle starter, the revolution of the rotation shaft is transmitted to an overrunning clutch through a reduction gear. In a starter of this construction, sometimes the armature core is reduced in size by increasing the reduction ratio of the reduction gear for miniaturization and reduction in weight of the dc motor. In this case also, the commutator size is physically determined, so that it cannot be reduced in size in correspondence with the armature core. Accordingly, the commutator becomes relatively larger than the armature core, and when the armature core is reduced in size, its revolution will be at a higher speed, hence a very large centrifugal force is imposed on the commutator.
In other applications not limited to vehicle starters the commutator segments are made larger than the armature core for improving a current-rectifying function. When the commutator is made large, brush sweep error is increased and the current density is reduced whereby the rectifying condition is improved and the brush service life is prolonged. In such a state also, the commutator will receive a larger force from the centrifugal force, since the commutator becomes relatively larger than the armature core.
Further, in every case described hereabove, when the commutator is of the so-called face type in which its contacting surface with brushes is formed perpendicular to the axial line of the rotation shaft, the size of the commutator directly manifests itself as an expansion of the dimension in the radial direction, resulting in an increase of the force it experiences. However, when a sufficiently large force due to centrifugal acceleration is exerted on the commutator, connection of the commutator segments by an insulator therebetween may be broken, presenting a problem of their bursting out.